Generally, a surveying instrument incorporates an inclination sensor, and is leveled up based on results of detection by the inclination sensor and adjusted so as to take a horizontal posture. As this inclination sensor, a liquid reflection type is commonly used. The liquid reflection type reflects a predetermined dark field pattern irradiated from a light source on a surface of a liquid (free liquid surface), receives this reflected light by a light receiving sensor, and detects a displacement amount of the light receiving pattern (light receiving image). For example, according to Patent Literature 1, a dark field pattern formed through one-dimensional slits is reflected on a free liquid surface, and a displacement amount thereof is detected. In Patent Literature 2, a dark field pattern formed through reticular two-dimensional slits is reflected on a free liquid surface, and the position of the pattern is calculated from a pattern array obtained by summing results of detection by a light receiving sensor in the X direction and the Y direction.